1. Field of the Invention
The present invention relates to froth flotation of solid materials and, more particularly, to a process and composition for the froth flotation of coal, molybdenum, graphite, and other materials having hydrophobic materials.
2. Description of Prior Art
When coal is mined, the raw product consists of coal and rock, with rock naturally occurring as small partings within the coal that cannot be avoided during the mining process. To concentrate the coal, large fragments of non-combustible mineral/matter are removed by screening or gravity separation techniques. Froth flotation is commonly used to beneficiate the finely divided raw coal. Certain coals such as Bituminous coals possess a natural hydrophobicity, which results in the coal being naturally floatable in the aqueous medium, but the use of reagents is still commonly required to enhance floatability and therefore recovery. Effective preparation of coal prior to combustion improves the homogeneity of coal supplied, produces less ash for disposal at power plants and other use sources, and reduces emissions of oxides of sulfur. Froth flotation/coal washing is an important method for reducing ash in coal. Washing the ash and coal is particularly critical for reducing sulfur, especially in coal fields in the Eastern United States.
During coal washing, the ore is crushed and wet ground to obtain a pulp. A frothing agent, usually employed with a collecting agent, is added to the coal/water slurry to effect the flotation. The coal slurry is aerated to produce froth at the surface thereof and the collector assists the frothing agent in separating the coal from the ore by causing the mineral values, i.e., the coal, to adhere to the bubbles formed during the aeration process. The portion of the ore which is not carried over with the froth is usually identified as flotation tailings or gangue and is disposed of or reprocessed. The purpose of the collector is to increase the hydrophobicity of the coal particles to permit better attachment to the bubbles which are considered hydrophobic. The purpose of the frothing agent is to stabilize bubbles and provide for a significant concentration of fine bubbles for attachment to the coal. The difference in density between the air bubbles and water provides buoyancy that preferentially lifts the hydrophobic solid particles to the surface where they remain entrained in the froth which can be drained off or mechanically skimmed away thereby effecting separation.
Collectors used in froth flotation of coal generally comprise hydrocarbon oils of which distillate oils such as kerosene, industrial diesel fuel and fuel oil are some of the most widely used, especially kerosene. While in the past collectors included hydrocarbons produced as coke-oven byproducts, the use of these materials has been largely discontinued since they contain phenols and other toxic aromatic hydrocarbons that pose ecological problems. Accordingly today the most widely used collectors are diesel or kerosene, i.e., middle distillate cuts. The problem with these middle distillate hydrocarbons is that they may contain significant amounts of polynuclear aromatics by up to 15% by wt., which result in increased levels of carcinogenicity and toxicity. Additionally these middle distillates, e.g., kerosene, diesel, contain significant concentrations of certain materials listed as “priority pollutants” (Clean Water Act). Diesel oil contains other HAPS listed materials such as benzene, toluene, ethybenzene, etc. in amounts ranging from about one-half to about 2% by wt. These materials are also highly toxic and pose problems with ground water contamination.